[PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

AKASHI Takahiro
2014-08-21T08:58:00Z

If tracer specifies -1 as a syscall number, this traced system call should
be skipped with a value in x0 used as a return value.
This patch enables this semantics, but there is a restriction here:

when syscall(-1) is issued by user, tracer cannot skip this system call
and modify a return value at syscall entry.

In order to ease this flavor, we need to treat whatever value in x0 as
a return value, but this might result in a bogus value being returned,
especially when tracer doesn't do anything at this syscall.
So we always return ENOSYS instead, while we have another chance to change
a return value at syscall exit.

Please also note:
* syscall entry tracing and syscall exit tracing (ftrace tracepoint and
audit) are always executed, if enabled, even when skipping a system call
(that is, -1).
In this way, we can avoid a potential bug where audit_syscall_entry()
might be called without audit_syscall_exit() at the previous system call
being called, that would cause OOPs in audit_syscall_entry().

* syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected
in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS)
is not used in this case, we may neglect the case.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
---
arch/arm64/include/asm/ptrace.h | 8 ++++++++
arch/arm64/kernel/entry.S | 4 ++++
arch/arm64/kernel/ptrace.c | 20 ++++++++++++++++++++
3 files changed, 32 insertions(+)

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 501000f..a58cf62 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -65,6 +65,14 @@
#define COMPAT_PT_TEXT_ADDR 0x10000
#define COMPAT_PT_DATA_ADDR 0x10004
#define COMPAT_PT_TEXT_END_ADDR 0x10008
+
+/*
+ * used to skip a system call when tracer changes its number to -1
+ * with ptrace(PTRACE_SET_SYSCALL)
+ */
+#define RET_SKIP_SYSCALL -1
+#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1)
+
#ifndef __ASSEMBLY__

/* sizeof(struct user) for AArch32 */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index f0b5e51..fdd6eae 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -25,6 +25,7 @@
#include <asm/asm-offsets.h>
#include <asm/errno.h>
#include <asm/esr.h>
+#include <asm/ptrace.h>
#include <asm/thread_info.h>
#include <asm/unistd.h>

@@ -671,6 +672,8 @@ ENDPROC(el0_svc)
__sys_trace:
mov x0, sp
bl syscall_trace_enter
+ cmp w0, #RET_SKIP_SYSCALL // skip syscall?
+ b.eq __sys_trace_return_skipped
adr lr, __sys_trace_return // return address
uxtw scno, w0 // syscall number (possibly new)
mov x1, sp // pointer to regs
@@ -685,6 +688,7 @@ __sys_trace:

__sys_trace_return:
str x0, [sp] // save returned x0
+__sys_trace_return_skipped: // x0 already in regs[0]
mov x0, sp
bl syscall_trace_exit
b ret_to_user
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 8876049..c54dbcc 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs,

asmlinkage int syscall_trace_enter(struct pt_regs *regs)
{
+ unsigned int saved_syscallno = regs->syscallno;
+
if (test_thread_flag(TIF_SYSCALL_TRACE))
tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);

+ if (IS_SKIP_SYSCALL(regs->syscallno)) {
+ /*
+ * RESTRICTION: we can't modify a return value of user
+ * issued syscall(-1) here. In order to ease this flavor,
+ * we need to treat whatever value in x0 as a return value,
+ * but this might result in a bogus value being returned.
+ */
+ /*
+ * NOTE: syscallno may also be set to -1 if fatal signal is
+ * detected in tracehook_report_syscall_entry(), but since
+ * a value set to x0 here is not used in this case, we may
+ * neglect the case.
+ */
+ if (!test_thread_flag(TIF_SYSCALL_TRACE) ||
+ (IS_SKIP_SYSCALL(saved_syscallno)))
+ regs->regs[0] = -ENOSYS;
+ }
+
if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
trace_sys_enter(regs, regs->syscallno);

--
1.7.9.5